ABSTRACT Synaptic vesicle cycling is highly dynamic and plastic, and subtle disruptions in its regulation may contribute to severe neurological disorders, including schizophrenia, epilepsy, Huntington's and Parkinson's Disease. Although a number of molecular determinants of vesicle cycling at synaptic terminals have been identified, our understanding of how vesicle cycling contributes to plasticity remains sparse. Using the Drosophila neuromuscular junction (NMJ), the lab of Dr. Bykhovskaia has discovered a pathway of presynaptic enhancement that may enable a nerve terminal to sustain and enhance its activity upon intense stimulation. Specifically, it was found that vesicle abundance in synaptic boutons is regulated by activity, and that intense stimulation increases vesicle number in nerve terminals. Importantly, these results suggest a functional implication for this form of plasticity: enhanced activity upon a subsequent intense stimulation. This application will employ a combination of molecular biology and genetics, live confocal imaging, and electron microscopy to unravel the mechanisms by which the terminal regulates its vesicle numbers in response to activity. This project will be performed in collaboration with the lab of Dr. Littleton at the Massachusetts Institute of Technology. Dr. Littleton is a leader in the field of Drosophila genetics and neurobiology, and thus this project will enable the Bykhovskaia lab to encompass Drosophila genetics approaches and to take the full advantage of this fruitful experimental preparation. Further, the project will test whether the phenomenon observed at the Drosophila glutamatergic synapse could be also detected in the mammalian central or peripheral synapses. Thus, the project combines multidisciplinary approaches at two genetic model organisms, mice and Drosophila, and involves a collaboration with the Massachusetts Institute of Technology, and as such it represents an excellent training vehicle for the graduate students from underrepresented groups. Dr. Bykhovskaia has a solid mentoring record, and the project will support two graduate research assistants, and thus it will contribute to the integration of research and training components.